In acoustic wave device technology a first metal layer is a patterned aluminum-based material layer known as an interdigital transducer (IDT) metal layer. Typically, an IDT metal layer comprises aluminum (Al) and a relatively smaller amount of titanium (Ti). The aluminum dominates the composition of the IDT metal layer because it has a higher ratio of conductivity to mass than titanium. The conductivity-to-mass ratio is a relatively critical characteristic for acoustic wave devices because a higher conductivity-to-mass ratio yields a lower insertion loss for radio frequency filters constructed from acoustic wave devices.
While aluminum is a desirable component of IDT metal, aluminum is not ideal because aluminum oxidizes readily when exposed to oxygen in air to form aluminum oxide. However, the oxidation of aluminum is typically self-limiting such that the formation of the oxide layer prevents additional oxidation of the aluminum metal making up the IDT metal layer. The oxide layer becomes detrimental at locations on the acoustic wave device where it is necessary to connect additional metal layers to the IDT metal layer.
In lithium tantalate surface acoustic wave (LTSAW) technology used to make a subset of acoustic wave devices, a second metal layer most often in contact with the IDT metal layer is an under bump metallurgy (UBM) metal. This second metal layer is deposited to form appropriate circuit connections for the acoustic wave device and is composed of Ti/Al/Ti. However, an aluminum oxide (AIO) layer formed on the IDT layer is insulating and is chemically and physically robust such that etching vias to connect the UBM to the IDT metal layer is not effective at removing the AIO layer. In addition to the robustness of the AIO layer, LTSAW process flow requires exposure of the aluminum IDT metal layer to ambient air during operations prior to UBM evaporation, which allows for re-oxidation of aluminum in any areas exposed by removal of the AIO during processing. The insulating nature of the AIO layer prevents consistent, low direct current (DC) contact resistance between UBM and the IDT metal layer. This AIO insulating layer also increases insertion loss by limiting conductivity between UBM and IDT layers. The lack of DC contact makes process control monitoring difficult because most process control monitoring is performed by DC measurement of test structures. The AIO between the UBM and the IDT metal layer also adds additional capacitance to radio frequency test structures, which can complicate parameter extraction and modeling. In addition to negative effects on electrical characteristics of acoustic devices, the AIO layer can also affect the mechanical properties of acoustic devices. The AIO layer prevents metal-to-metal contact between the IDT metal layer and the UBM. Therefore, adhesion between the IDT metal layer and the UBM is reduced. This reduction in adhesion reduces shear strength between the IDT metal layer and the UBM. As such, the robustness of acoustic devices during assembly is reduced by mechanical stresses of assembly. Thus, there is a need for acoustic devices that do not have AIO layers formed between the UBM and IDT layers.